Power fault battery protection circuit

ABSTRACT

A battery protection circuit is provided that includes a safety circuit and an overpower circuit. The safety circuit monitors the voltage and current of at least one rechargeable cell within the battery pack, and disconnects the cell(s) from the external terminals of the battery pack when either the voltage becomes too high or low, or when excessive current is being drawn from the battery pack. The overpower circuit monitors the power being delivered to or sourced from the battery pack to the load. The overpower circuit actuates when the power exceeds a predetermined threshold, thereby simulating an overcurrent condition in the safety circuit. The overcurrent condition causes a disconnect means, like a transistor, to open, thereby disconnecting the cell(s) from the external terminals. The battery protection circuit then latches in this disconnected state until a load is removed from the terminals of the battery pack.

BACKGROUND

1. Technical Field

This invention relates generally to protection circuits for rechargeablebattery packs, and more specifically to protection circuits that disablea rechargeable battery pack due to an excessive amount of power beingdrawn by the load.

2. Background Art

Portable electronic devices, like cellular telephones, pagers andtwo-way radios for example, derive their portability from rechargeablebatteries. Such batteries allow these devices to slip the surly bonds ofwall mounted power supplies and wirelessly touch the hand of the userwherever he may be.

While many people may think that a rechargeable battery is simply a celland a plastic housing, nothing could be further from the truth.Rechargeable battery packs often include circuit boards, electroniccircuitry, mechanical assemblies and electromechanical protectioncomponents. The circuits employed in rechargeable battery packs includecharging circuits that start, ramp, taper and stop current, fuel gaugingcircuits, temperature measurement circuits and indicator circuits, justto name a few. Simply put, a battery pack is a complex system ofcomponents working in harmony to safely deliver power to a portableelectronic device.

One of the most fundamental circuits in a battery pack is the protectioncircuit. Rechargeable battery performance, especially with respect tothose having cells constructed of lithium-based materials, may beseverely compromised if the cell within the battery pack is over orunder charged. For this reason, most all battery packs today include oneform of safety circuit or another.

Typical safety circuits include voltage and current limits. As such,when the voltage across the cell in a battery pack becomes too high ortoo low, the safety circuit will open switches within the pack, thereby“turning off” the battery pack. Similarly, if the current flowing eitherinto or out of the cell gets too high, the safety circuit will turn offthe battery pack.

Despite these voltage and current safety mechanisms, new concerns arearising from “over power” situations. These situations arise when abattery pack is operating within its voltage and current limits, but thetotal power—the product of voltage and current—becomes too high for aparticular electronic device. The concern is that the over powersituation may cause components within the electronic device to generateexcessive heat.

There is thus a need for an improved battery safety circuit that turnsoff the battery not only due to excessive voltage or current, but forexcessive power dissipation as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a safety circuit IC.

FIG. 2 illustrates a protection circuit having a safety circuit andoverpower circuit in accordance with the invention.

FIG. 3 illustrates one embodiment of a power meter in accordance withthe invention.

FIG. 4 illustrates one embodiment of an analog multiplier in accordancewith the invention.

FIG. 5 illustrates a protection circuit having a plurality of safetycircuits and overpower circuits in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. As used in the description herein and throughout the claims,the following terms take the meanings explicitly associated herein,unless the context clearly dictates otherwise: the meaning of “a,” “an,”and “the” includes plural reference, the meaning of “in” includes “in”and “on.”

This invention provides a overpower protection circuit that may be usedin conjunction with an existing battery safety circuit to offer anadditional level of protection. The overpower circuit monitors the powerbeing delivered to or from the cells in a battery pack. When the powerexceeds a predetermined threshold, the overpower circuit simulates anovercurrent condition with respect to the safety circuit. Thisovercurrent simulation causes the protection circuit to open one or moreserial pass elements, thereby isolating the cells from the externalterminals of the battery pack. The overpower circuit then resets itselfwhen the load is removed from the battery pack. The combination of theoverpower circuit with the conventional battery safety circuit providesa system that protects not only from excessive voltages and currents,but from excessive power dissipation levels as well.

Prior to understanding the overpower circuit, a brief overview ofbattery safety circuits is warranted. As used herein, a “safety circuit”is any circuit capable of monitoring the voltage across at least onerechargeable cell, in addition to being capable of monitoring thecurrent flowing through the cell or cells. One example of such a circuitis the S8232 series of safety circuits manufactured by SeikoInstruments, Inc. For discussion and exemplary purposes, such a circuitwill be discussed herein. It will be clear to those of ordinary skill inthe art who have the benefit of this disclosure, however, that theinvention is not so limited. Discrete circuits, application specificcircuits and safety circuits manufactured by other companies, includingRicoh and Mitsumi for example, may be equally substituted for the Seikocircuit.

By way of background, referring now to FIG. 1, illustrated therein is ablock diagram of an S-8232 safety circuit 100. The S-8232 safety circuitis designed to be used with two, serial, lithium-based cells. Again, itwill be clear to those of ordinary skill in the art with the benefit ofthis disclosure that the invention is not so limited. The overpowercircuit discussed herein may be equally applied to any combination ofserial or parallel cells.

The safety circuit 100 may be as simple as a single integrated circuit(IC) that provides a means for monitoring of cell voltage and current,and thereby controls the charging and discharging of the cells within abattery pack. Discrete equivalents of the IC may also be substituted.The safety circuit 100 includes an overcharge detector 101 that monitorsthe voltages across the corresponding cells. The overcharge detector 101compares these voltages to a predetermined maximum cell voltage. Whenthe cell voltage exceeds this threshold, the overcharge detector 101,via some internal logic circuitry 103, causes a push-pull output stage114 to actuate the charge pin 107. When the charge pin 107 is coupled toa disconnect means, like a transistor acting as a switch in itsnon-linear region, actuation will prevent any further charging of thecells.

Similarly, the safety circuit includes an overdischarge detector 102that ensures that the voltage across the cells does not fall below apredetermined threshold. If it does, the overdischarge detector 102causes an output stage 113 to actuate the discharge pin 106. When thedischarge pin 106 is coupled to a disconnect means, like a serialtransistor, actuation prevents any further discharge.

Cell current is monitored by way of an overcurrent detection pin 108coupled to an overcurrent detection circuit 104. The overcurrentdetection pin 108 senses the voltage between the Vss pin 112 and theovercurrent detection pin 108. When this voltage exceeds a predeterminedthreshold, as will be explained in more detail later, the overcurrentcircuit 104 causes the discharging pin 106 to actuate, thereby stoppingthe flow of current in the discharge direction. In some situations, withsome safety circuits, the charging pin 107 may also actuate.

When the load is removed, as evidenced by an impedance greater than 200MΩ appearing between the Sens pin 110 and the overcurrent pin 108, thesafety circuit 100 resets, thereby deactuating the discharge pin 106.This action will be more evident with the discussion of FIG. 2 below.

Other components of the safety circuit 100 include a Vcc pin 109, acenter tap pin 111, and a Vss pin 112, that monitor the voltage at thecathode, between, and at the anode of serial cells, respectively.Additionally, a delay circuit 105 provides some hysteresis and transientimmunity.

Referring now to FIG. 2, illustrated therein is one preferred embodimentof a battery protection circuit in accordance with the invention. Thesafety circuit 100 from FIG. 1 is coupled to a pair of rechargeablecells 201, 202. The charge pin 107 and the discharge pin 106 are coupledto disconnect means 203, 204, respectively, which are in turn coupledserially with the cells 201, 202. The disconnect means 203, 204 in thisexemplary embodiment are field effect transistors (FETs), although otherdevices, including switches, relays, circuit breakers and controllablefuses may be substituted, depending upon the application.

The overcurrent pin 108 is coupled to the low side 205 of-the circuit,such that the overcurrent pin 108 may work in conjunction with the Vsspin 112 to sense the voltage across the FETs 203, 204. When this voltagebecomes too high, the safety circuit 100 knows that the current beingdrawn from the cells 201, 202 is correspondingly too high. When thisoccurs, the discharge pin 106 causes the FETs 203 to open, therebypreventing current from flowing to the external terminals 206, 207. Thesafety circuit 100 resets, and thus closes the FET 203, when animpedance greater than 200 MΩ is sensed between the Sens pin 110 and theovercurrent pin 108. This occurs when a load (not shown) is removed fromthe terminals 206, 207, thereby creating an open circuit between theterminals 206, 207.

The overpower circuit 208 includes a power meter 209 that acts as ameans of monitoring power being delivered to or from the cells 201, 202.The power meter 209, explained in more detail with the discussion ofFIG. 3, is any circuit that is capable of determining whether theproduct of the voltage across the at least one rechargeable cell and thecurrent flowing through the at least one rechargeable cell exceeds thepredetermined threshold. It may include a circuit that generates asignal that is proportional to the product of the voltage across thecells and the current flowing through the cells. It may also be acircuit that simply generates a binary, up or down signal that indicateswhether the power is above or below the threshold. The power meter 209may be an accurate, linear power meter, or may be a simpler circuit thatapproximates power, for example by way of piecewise linear or otherapproximation means.

The power signal 210 is then coupled to a comparator 211 that has asignal 212 (like a reference voltage) that is proportional to thepredetermined threshold of power. When the power signal 210 exceeds thepredetermined threshold 212, the comparator 211 actuates. Thepredetermined threshold may be set to any level required by theapplication. One exemplary threshold for a two-serial-cellconfiguration, that is intended to comply with a correspondingtemperature threshold limit set forth by the Atmospheric Explosive(ATEX) directive set forth by the European Union, is nine watts. In anyevent, when the signal proportional to power is below the predeterminedthreshold, the output 216 of the comparator 211 is in a first state. Theoutput 216 of the comparator 211 switches to a second state when thesignal proportional to power exceeds the predetermined threshold.

When the power sourced from the cells 201, 202 exceeds the predeterminedthreshold, the overpower circuit 208 simulates an overcurrent conditionwithin the safety circuit 100, causing the FET 203 to open or enter ahigh impedance state, thereby preventing current from flowing from thecells 201, 202. The overcurrent condition is simulated by sourcingcurrent into the overcurrent pin 108 (as a result of increased voltageat the overcurrent pin 108), and thus into the overcurrent detectioncircuit within the safety circuit 100.

Such an overpower condition would arise as follows: The power meter 209would be continually monitoring the power sourced from the cells 201,202. The load connected to the terminals 206, 206 would begin drawingpower in excess of the predetermined threshold. The power meter 209determines that this is the case, causing the power signal 210 to riseabove the power threshold signal 212. This, in turn, actuates thecomparator 211.

A switch 219, shown here as a FET, is responsive to the comparator andcloses upon actuation of the comparator 211. This pulls the overcurrentpin 108 to the cell voltages, thereby causing current to flow into theovercurrent pin 108 through a current limiting resistor 213. To thesafety circuit 100, this appears to be an overcurrent situation. Thesafety circuit 100 then opens the discharge FET 203, thereby preventingany current from flowing out of the cells 201, 202. As such, the cells201, 202 are disconnected from the terminals 206, 207 as a result ofpower dissipation exceeding the predetermined threshold. An optionaldelay circuit 218, for example a R-C filter, may be coupled between thecomparator 211 and the switch 219 where a delay prior to opening the FET203 is desired. Such a delay may be desirable when a host device needstime to complete an operation prior to power down.

In parallel, an optional second disconnect means 214, shown here as aFET, may be coupled to the comparator 211 so as to be responsive to thecomparator 211. The second disconnect means 214, coupled seriallybetween the terminals 206, 207 and the cells 201, 202, operates as asecondary circuit breaker and opens when the comparator 211 is actuated.As such, if the discharge FET 203 fails, the second disconnect meanswill still disconnect the cells 201, 202 from the terminals 206, 207when an overpower condition occurs.

Note that this second disconnect means 214 is optional, as it isadvantageous in some designs. For example, in circuits where redundancyis needed for reliability, a designer may decide to employ two separatesafety circuits, using the second safety circuit to control a seconddischarge FET, which would thus serve as the optional second disconnectmeans. In such a case, one example of which is illustrated in FIG. 5,either the overpower circuit 208, or a redundant overpower circuit 501,would be connected to the overcurrent pin 502 of a second safety circuit503, with the charge and discharge FETs 504, 505 of the second safetycircuit 503 being coupled serially with the terminals 206, 207, thecells 201, 202, and the first charge and discharge FETs 203, 204. Otherdesigns may need neither the second safety circuit nor the seconddisconnect means.

A leakage current path in parallel with the second disconnect means 214is provided by a resistor 215. The resistor 215 provides a latchingmechanism that causes the safety circuit 100 to remain in the simulatedovercurrent condition. Recall that the safety circuit 100 resets whenthe impedance between the sense pin 110 and the overcurrent pin 108exceeds 200 MΩ. This would be the case when the second disconnect means214 opens. As such, a leakage path provided by the resistor 215 ensuresthat the safety circuit 100 stays latched in the simulated overcurrrentcondition until the load is removed from the terminals 206, 207.Resistance values for resistor 215 range from 100 kΩ to 500 kΩ,preferably about 200 kΩ. Note that when a second safety circuit is usedto control the second disconnect means 214, the leakage current path maynot be necessary, as the second safety circuit provides an internalleakage path.

As stated, when the overpower, and thus the simulated overcurrent,condition is initiated, the cells 201, 202 are disconnected from theterminals 206, 207 by way of the FET 203. In such a state, it is notdesirable to have electrical components within the battery packdischarge the cells 201, 202. As such, the invention provides a means ofdisabling the overpower circuit 208. Disablement is accomplished by aswitch coupled between the high side terminal 206 of the circuit and theoverpower circuit 208. This switch 217, shown for exemplary purposes asa FET, is coupled to the discharge pin 106 of the safety circuit 100.

When the discharge pin 106 is actuated, the switch 217 turns off,thereby blocking current from flowing to the overpower circuit. Thus, inan overpower condition, the overpower circuit 208 first simulates anovercurrent condition in the safety circuit 100, thereby causing thedischarge pin 106 to actuate. This, in turn, causes the switch 217 toopen, thereby deactivating the overpower circuit 208.

Such a scenario is perfectly acceptable, in that the overpower circuit208 is no longer needed to monitor power being delivered from the cells201, 202, as there is no power being delivered from the cells 201, 202since the charge and discharge FET 203 is open. Upon removal of theload, however, the safety circuit 100 resets, thereby causing closure ofthe FET 203, thereby closing the switch 217, thereby reactivating theoverpower circuit 208. The safety circuit will then revert back tonormal operation.

Referring now to FIG. 3 illustrated therein is one example of a powermeter 209 in accordance with the invention. The power meter 209 includesa means for measuring or sensing the voltage across the cells, as wellas a means for measuring or sensing the current flowing through thecells. Both the means of measuring voltage and current may compriseanalog amplifiers 301, 302 coupled to the cells. In the case of voltage,the amplifier 301 may have inputs coupled across the cells to measurethe voltage. The gain of the amplifier 301 would be scaled such that theproduct output is at a level that is acceptable by the comparator.

In the case of current, the amplifier 302 input may be coupled to ameans of sensing current, like a current sense resistor for example. Aswith the voltage amplifier 301, the gain of the current amplifier 302would be scaled such that the product output is at a level that isacceptable by the comparator.

The outputs of the amplifiers 301, 302 are then fed into an analogmultiplier 303. The analog multiplier produces a product output 304 thatis proportional to the product of voltage and current. This output 304is then fed to the comparator 211. One example of an analog multiplieris shown in FIG. 4, and is also taught in U.S. Pat. No. 3,562,553,entitled “Multiplier Circuit, issued to Roth, which is incorporatedherein by reference for all purposes.

Note that the power meter of FIG. 2 and the multiplier circuit of FIG. 3are but one exemplary embodiment of a power meter in accordance with theinvention. It will be clear to those of ordinary skill in the art whohave the benefit of this disclosure that the invention is not solimited. Numerous other power measurement circuits, including thoseemploying logarithmic amplifiers, microprocessors with analog to digitalconverters, hall effect multipliers, and other analog and digitalcircuits may be equally substituted. The only requirement is that thepower measurement circuit be capable of producing a signal proportionalto the amount of power being sourced from, or delivered to, the cells.

While the preferred embodiments of the invention have been illustratedand described, it is clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions, andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as defined by thefollowing claims. For example, while one preferred embodiment of theinvention is a rechargeable battery pack comprising the batteryprotection circuit taught in FIG. 2, the invention is not so limited. Itmay be applied to any power source, including power supplies, fuelcells, solar cells and the like. Additionally, it may be incorporatedinto the host device as well as within the battery pack.

1. A battery protection circuit, comprising: a. at least onerechargeable cell; b. a safety circuit coupled to the at least onerechargeable cell, the safety circuit comprising a means for monitoringa voltage across the at least one rechargeable cell, further comprisinga means for monitoring a current flowing through the at least onerechargeable cell; c. at least one disconnect means coupled seriallywith the at least one rechargeable cell; d. a means of monitoring powerbeing delivered either to or from the at least one rechargeable cell;and e. a means for simulating an overcurrent condition within the safetycircuit when the means for monitoring power determines that the powerbeing delivered either to or from the at least one rechargeable cellexceeds a predetermined threshold.
 2. The circuit of claim 1, whereinthe means of monitoring power monitors the power being delivered fromthe at least one rechargeable cell.
 3. The circuit of claim 2, furthercomprising at least a second disconnect means, the at least a seconddisconnect means being responsive to the means for monitoring powerbeing delivered from the at least one rechargeable cell.
 4. The circuitof claim 3, further comprising a leakage current path in parallel withthe at least a second disconnect means, the leakage current path havinga resistance in excess of one hundred thousand Ohms.
 5. The circuit ofclaim 1, further comprising at least an additional disconnect meanscoupled between the at least one rechargeable cell and the means ofmonitoring power delivered to or from the at least one rechargeablecell, wherein when the overcurrent condition is simulated, the safetycircuit actuates the at least an additional disconnect means todeactivate the means for monitoring power delivered to or from the atleast one rechargeable cell.
 6. The circuit of claim 1, wherein the atleast one disconnect means arc selected from the group consisting oftransistors, switches, relays, circuit breakers, and fuses and positivetemperature coefficient devices.
 7. The circuit of claim 1, wherein themeans for monitoring power being delivered comprises: a. a means formeasuring the voltage across the at least on rechargeable cell; b. ameans for measuring the current flowing through the at cast onrechargeable cell; c. a means of determining whether the product of thevoltage across the at least one rechargeable cell and the currentflowing through the at least one rechargeable cell exceeds thepredetermined threshold, and d. a comparator, wherein an output of thecomparator is in a first state when a product of the voltage across theat least one rechargeable cell and the current flowing through the atleast one rechargeable cell is below the predetermined threshold;further wherein the output of the comparator is in a second state when aproduct of the voltage across the at least one rechargeable cell and thecurrent flowing through the at least one rechargeable cell is above thepredetermined threshold.
 8. The circuit of claim 1, wherein thepredetermined threshold is nine watts.
 9. The circuit of claim 1,wherein the safety circuit comprises: a. an overcharge detector; b. anundercharge detector; and c. an overcurrent detection circuit.
 10. Thecircuit of claim 9, wherein the overcurrent situation is simulated bysourcing current into the overcurrent detection circuit.
 11. Arechargeable battery pack comprising the circuit of claim
 1. 12. Abattery protection circuit having a power monitoring circuit, whereinthe power monitoring circuit determines when power exceeds apredetermined threshold, comprising: a. at least one rechargeable cell;b. at least one safety circuit coupled to the at least one rechargeablecell; c. at least one disconnect means coupled serially with the atleast one rechargeable cell; and d. at least one means for simulating anovercurrent condition within the safety circuit when the powermonitoring circuit determines that power exceeds the predeterminedthreshold.
 13. The circuit of claim 12, wherein when the at least onemeans for simulating an overcurrent condition within the safety circuitsimulates an overcurrent condition, the at least one disconnect meansenters a high impedance state.
 14. The circuit of claim 13, wherein theat least one disconnect means are selected from the group consisting oftransistors, switches, relays, circuit breakers, and fuses and positivetemperature coefficient devices.
 15. The circuit of claim 14, whereinpower monitoring circuit comprises: a a means for sensing the voltageacross the at least on rechargeable cell; b. a means for sensing thecurrent flowing through the at least on rechargeable cell; c. a means ofdetermining whether the product of the voltage across the at least onerechargeable cell and the current flowing through the at least onerechargeable cell exceeds the predetermined threshold; and d. acomparator, wherein an output of the comparator is in a first state whena product of the voltage across the at least one rechargeable cell andthe current flowing through the at least one rechargeable cell is belowthe predetermined threshold; further wherein the output of thecomparator is in a second state when a product of the voltage across theat least one rechargeable cell and the current flowing through the atleast one rechargeable cell is above the predetermined threshold. 16.The circuit of claim 15, wherein the predetermined threshold is ninewatts.
 17. The circuit of claim 16, wherein the safety circuitcomprises: a. an overcharge detector; b. an undercharge detector; and c.an overcurrent detection circuit.
 18. The circuit of claim 17, whereinthe overcurrent situation is simulated by sourcing current into theovercurrent detection circuit.